Mechanical Impedance tuners are widely used in characterization of RF devices. The most common type of impedance tuner is the slab line tuner, which typically includes a 50 Ohm TEM slab line, with a movable mismatch probe. FIGS. 1A and 1B illustrate such a tuner in simplified form, with a center conductor 10 supported between opposed conductive ground planes 12, 14 which form the slab line. A mismatch probe 20 is supported for movement along or horizontal to the center conductor, and also in a direction traverse to or vertical to the center conductor. If the mismatch probe is moved out of the electrical fields of the slab line, it has almost no effect, allowing the slab line to look like a good 50 Ohm (characteristic impedance) line. However, if the mismatch probe is moved close to the center conductor, the electrical fields are affected, causing a mismatch. The magnitude of the mismatch is controlled primarily by adjusting the distance of the probe from the center conductor. The phase of the mismatch is controlled by moving the probe in a direction parallel to the center conductor. A similar effect can also be done in other transmission line environments, such as waveguide.
Mismatch probes have a frequency response based on their geometry. Here, the frequency response of the probe is in the context of the probe being mounted appropriately in the tuner. In principle, longer probes will have a “flat” operating response at lower frequencies, whereas shorter probes will have a “flat” operating response at higher frequencies. In this document, the “flat” operating response of a probe is defined to be a reflection response that occurs in the frequency range where the maximum reflection (or VSWR) is equal or greater than some specified value.
Typical impedance tuners use one, two, or three mismatch probes in a single carriage in order to increase the overall operational frequency bandwidth of the tuner.